The operative aspects of an optical fiber typically comprise an inner core and a cladding layer surrounding the core. For most (if not all) applications, it is also desirable to provide a protective coating around the cladding or to encase the fiber within a tight-fitting tube. The protective coating is important for protecting the fiber from surface damage caused by handling or exposure to deleterious environments and reducing microbending losses. Typical materials used for coating optical fibers involve polymeric materials such as nylon, polypropylene, polyurethane, or kynar, a vinylidene fluoride polymer commonly used as an electrical insulator.
Customarily with prior art methods, the coating of the optical fiber is performed by passing the fiber through a bath of molten or liquid polymer (or other material used for the coating), and then curing the polymer after the fiber is removed from the bath by, for example, irradiation or passing the fiber through a furnace or other curing means. Such a prior art method is described below with reference to FIG. 1 hereof, which illustrates a customary method for simultaneously manufacturing and coating an optical fiber and wrapping it on a take-up drum.
With such methods, it is naturally desirable to increase the speed at which the fiber is passed through the polymer bath, because this will increase the efficiency and production output of not only the coating operation but also of the entire system. As the draw speed is increased, however, there is an increased tendency for air bubbles to be entrapped within the coating during the coating operation. Air bubbles can be problematic as they can affect the integrity of the coating and offer a capsule for collection of vapor or ambient contaminants. Additionally, air bubbles on or within the coating can induce or contribute to the formation of small deformations or "microbends" on the surface of the fiber; such microbends may affect the angles at which rays within the fiber reflect and lead to significant optical losses.
A previous method of dealing with the tendency of the optical fiber coating system to create air bubbles when the draw speed is increased involves the use of a clever die design and pressurization technique. This technique is premised on the principle that, when the draw speed is increased, the increased friction caused by the fiber passing through the liquid polymer tends to draw the polymer toward the bottom surface of the bath, thus causing the formation of an increased negative meniscus in the bath container and the drawing of additional air into the polymer bath. Therefore, with the pressurization technique, the pressurization of the die is used to form a positive (convex) meniscus of the liquid relative to the coating container to counteract the negative (concave) meniscus formed by the drawing of the fiber through the polymer bath.
There are, however, limitations to the pressurization technique, especially for higher draw speeds. Most particularly, the response time for creating the positive meniscus with this technique is slow, making it difficult to effectively adjust the rate of operation of the system in real time. Also, especially for high speed operations the speed of the system needs to be ramped-up slowly. Applying the pressurization technique, adjustments in the rate of speed of the system are difficult to effectively achieve and maintain.
The present invention is, therefore, addressed to a method of coating an optical fiber in which the fiber can be quickly drawn through a bath of coating material while minimizing the formation of air bubbles in the coating. The invention further embraces an apparatus for performing this method. Further advantages of this invention may appear more fully upon consideration of the detailed description below.